Entomologists have a saying, “there is always one more bug!”. With nearly one million classified species of insects so far and another estimated 30 million left to be discovered, every season is new and exciting. In the case of the elm flea weevil, it is no exception.
The elm flea weevil (Orchestes alni) first showed up in Illinois and Wisconsin in the summer of
2003. Because of its abundance and unique characteristics, it immediately received the attention of entomologists, nursery growers, arborists, urban foresters, landscape managers, and other green industry members. It reappeared again in summer, 2004 and in some locales at levels higher than in 2003.
In this article, I will provide an update on pest identification and biology, host preference,
chemical and non-chemical management and potential for damage.
The elm flea weevil (EFW) was unknown here in the Midwest until summer, 2003 when it
showed up feeding on elms in both Illinois and Wisconsin. According to insects taxonomists, it is quite common in Europe and was reported in northeastern North America in 1982. Presently it is found Illinois and Wisconsin. We first noticed it at The Morton Arboretum feeding on Chinese elm, Ulmus parvifolia. There are very few insects that feed on U. parvifolia and at first we thought it was leaf tatter damage. After further examination, we noticed tiny little “beetles” in our hair and around our shoulders and neck as they dropped down from the tree canopy. In addition, we observed similar feeding damage on other elm species most notably, Siberian elm, U. pumila.
The adult EFW is very tiny (1.6 mm long), dark brown to black, with a prominent proboscis or
“beak” making it a member of the weevil family of beetles. Flea weevils also have an enlarged
femur on the hind leg similar to a flea’s leg hence the name flea weevil. They jump when
disturbed. Since not much is known about the elm flea weevil biology, I will provide information
on related flea weevil species.
Overwintering EFW adults first appear in May and early June and begin feeding on the
undersides of newly emerging foliage, primarily elms. Adult feeding results in tiny shot holes in the leaves. Heavy feeding can cause newly expanding leaves to wither and turn brown. After
feeding, the female weevil cuts a cavity into the leaf mid-vein and inserts an egg. Larvae act like leafminers initially mining the leaf-tip, but as feeding continues, the mine becomes a blotch. Mined leaves will appear scorched by mid to late summer, depending on weather conditions. The larvae are dirty white, widest at the thorax, and about 12 mm (˝ inch) long when mature. The larvae will feed for about 2-3 weeks and then pupate within the mined leaf. Adults emerge in late July or early August. Adults appear to overwinter under loose bark and in ground litter under and near infested trees. A related weevil species, the apple flea weevil has one generation, but the juryis still out on the elm flea weevil. Nursery growers and consulting entomologists report seeing adults all season long with heavy populations early in the summer. Further research is needed to better define the life cycle and habits of this new pest.
As for preferred hosts, 2003 and 2004 field defoliation surveys and observations taken at The
Morton Arboretum and by others indicate that Siberian elm U. pumila and simple and complex
hybrids containing U. pumila, particularly “Homestead” elm are heavily fed upon. These results
are consistent with other reports for Illinois and Wisconsin.
Now, if the EFW shows up, what is its potential for damage and how do we manage it? The EFW definitely has the potential to seriously defoliate elms, particularly the preferred hosts mentioned above. Defoliation will not kill the tree directly, but with any defoliating insect, the tree may become weaken and/or stressed predisposing it to lethal borers and pathogens. Since this insect tends to defoliate early in the growing season, defoliation can have a major impact on photosynthetic capacity of the tree impacting the overall vitality of the tree. Most of the inquiries that were received came from homeowners and green industry professionals due to very extensive and heavy feeding damage.
An important aspect of this pest is proper identification and plant diagnosis. In spring 2004, many parts of the Midwest were quite dry in April when leaf emergence and leaf expansion was occurring. A few tree species that were leafing out at the time had leaves about half their normal size. In addition, in some areas, we experienced near or below freezing temperatures in earlyMay, 2004. Putting these two abiotic factors together, along with early EFW feeding damage, many elm trees were in poor leaf form well into early to mid summer. Later in the summer, the heavily mined leaves resembled leaf scorch. Be sure to remember to carefully look at the leaf when diagnosing leafminer damage. Leaves damaged by leafmining insects will leave behind frass and head capsules in the leaf whereas these signs will be absent with leaf scorch. In addition, leaf scorch symptoms usually begins at the leaf margins and progresses inward. Obviously, weather plays a role here with hot, dry weather being the major culprit.
As for EFW management, most trees will recover by implementing good plant health care
practices including proper watering, fertilizing, pruning, and mulching. On high value trees and
trees under stress, chemical management may be needed. Spraying for adults as soon as they appear can be effective. Some growers and managers are coupling this with a systemic insecticide to control the mining larvae. Depending on the residual of the insecticide, several applications may be needed during the course of the summer. Additional field research is needed here to determine the proper insecticide regimen and most economical way to deal with this pest.
Keep a look out for the elm flea weevil and make sure you properly identified it and the damage it causes. 2005 is a new year and will bring new challenges for all of us who grow and maintain plants.